Most cars are now equipped with a power-assist or power steering device for reducing the torque exerted on the steering wheel to modify the turn angle of the front direction wheels.
The most widely used power-assist device is hydraulic, and comprises a hydraulic actuator, which generates a power-assist torque and is controlled by a number of valves activated by the angular position of the steering wheel.
The major drawback of power-assist devices of the above type lies in the power-assist torque being constant, and so tending to “overslacken” the steering wheel at high speed, so that the driver is unable to actually feel the dynamic performance of the car. To eliminate this drawback, it has been proposed to equip the power-assist device with a speed-sensitive valve, which gradually reduces the power-assist torque as speed increases.
Even a power-assist device with a speed-sensitive valve, however, does not allow the driver to accurately feel the dynamic performance of the car, especially when the car is equipped with electronic dynamic-performance control devices which greatly reduce the reactions of the car. For example, an ASR device prevents spinning of the drive wheels; an ABS (Anti Block System) and MSR device prevent the wheels locking when braking; an E-diff (electronic differential lock percentage control) device and an ESP (Electronic Stability Program) device limit swerving and excessive load transfer.
DE10245975A1discloses a controlled planetary steering transmission with hydraulic rack assistance. The steering system comprises a hydraulic assistance computing unit controlling a valve 2 in response to signals including sensed input torque, and a steering wheel delivering the input torque to a portion of a steering column, from which rotation is transmitted via epicyclic gearing and a pinion to the rack.